The present invention relates generally to vehicle temperature control systems and specifically to a temperature control system for a fuel cell vehicle.
In an effort to find new energy sources, fuel cells using an electrochemical reaction to generate electricity are becoming an attractive energy alternative. Fuel cells offer low emissions, high fuel energy, conversion efficiencies, and low levels of noise and vibration. U.S. Pat. No. 5,248,566 to Kumar et al. These advantages make fuel cells useful in automotive applications. Of the various types of fuel cells, the proton electrolyte membrane (PEM) fuel cell appears to be the most suitable for use in automobiles, as it can produce potentially high levels of energy, but has low weight and volume.
Fuel cells, while simple in concept, are quite complex since many systems need to be considered to operate the fuel cell optimally. One of the design challenges of a vehicle with a PEM fuel cell stack is the high volume of heat it produces while in operation. Thermal management systems (cooling systems) are known for conventional internal combustion engine (ICE) vehicles. Unfortunately, fuel cell powered vehicles have unique thermal management requirements when compared to ICE vehicles. One of these requirements is that the aqueous-based coolant must be maintained at a specific temperature at the fuel cell stack inlet. At the same time, the inlet temperature of a separate, so-called low temperature cooling circuit must be limited to a specific temperature. Further, a temperature rise of the coolant across the stack needs to be maintained at a precise value. These requirements are needed to maintain the correct fuel cell stack efficiency and humidification that in turn effect the performance and durability of the stack.
Fuel cell thermal management systems are known in the art, but none appear to have dual controls for coolant temperature. A new fuel cell cooling system control strategy needs to be developed to address the deficiencies of the prior art. This new thermal management system for the fuel cell should include a cooling circuit with variable fan speed control and flow control actuators such as a radiator bypass valve, to deliver the precise temperature controls needed.
Accordingly, the present invention is a system and method to control coolant temperature within a fuel cell vehicle. Coolant inlet temperature is controlled by adjusting system fan speed and a radiator bypass valve position. For this invention, a general control solution with independent controllers for fan and bypass valve operating modes is proposed. The controllers can have different parameters and structures so they can provide better control performance for significantly different plant responses with respect to fan speed and bypass valve position plant inputs. For this invention, a plant involves what the controller is trying to control. The controllers are subsequently activated using simple and robust coordination logic. An additional control and coordination algorithm that provides limitation of the inlet temperature for the low temperature cooling circuit extends this control approach.
Further, inlet temperature control error is highly influenced by change of the net power and vehicle speed disturbance variables. The controller is extended by a feed-forward disturbance compensator, to mitigate the influence of the disturbances to improve the control system""s performance. This improves performance in that rather than reacting to changes in coolant temperature, the controller is able to anticipate these changes before they occur. Gain scheduling is used to improve the performance of the feed-forward compensator that includes dependence on ambient temperature.
Further, fuel cell stack behavior in one mode (in which the fuel cell coolant inlet temperature is controlled by the operation of a radiator bypass valve) is characterized by a distinctive dead-zone nonlinearity, especially at low vehicle speeds. This dead-zone effect results in a large inlet coolant temperature dynamic control error during a significant portion of the radiator bypass valve""s operation. Another disturbance effect is also caused by the abrupt change of fan speed requested from air-conditioning (A/C and cooling systems use the same fan for working fluid pressure and/or temperature control). The control strategy is extended by a controller-preset algorithm that efficiently compensates for the valve operation nonlinear effects and for A/C fan operation disturbance effects.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.